The present invention relates to an electrophotographic copying apparatus and more particularly, to an electrophotographic copying apparatus equipped with an improved variable magnification arrangement.
It has been a recent trend that electrophotographic copying apparatuses, for example, of the slit exposure type, in which either a platform to hold an original to be copied thereon or an optical system is adapted to move for exposing a photosensitive image forming surface to light images of the original in a known manner, are provided with a variable magnification arrangement mainly for copying of the original on a reduced scale for efficient business transactions and filing of copied documents and the like.
Commonly, for the projection of light images of the original onto an image forming surface through a lens or lens assembly, the following relations are established. ##EQU1## where am represents the length of the light path between the original and the lens assembly, bm denotes the length of the light path between the lens assembly and the image forming surface, f is the focal length of the lens assembly, and m is the projecting magnification represented by (bm/am).
The length lm of light path between the original and image forming surface, i.e. the conjugate distance, is represented by ##EQU2## as is well known to those skilled in the art.
In connection with the above, if the projecting magnification m is varied, the length am of the light path between the original and lens assembly, the length bm of the light path between the lens assembly and the image forming surface, and the conjugate distance lm are naturally altered accordingly.
Therefore, in the optical systems of the conventional copying apparatuses, it has been a general practice for varying magnifications to move the position of the lens assembly and simultaneously to alter the position of the original or that of the reflecting mirror provided on the light path for the variation of the length of the light path between the original and the surface on which the image of the original is to be projected.
The conventional arrangement as described above, however, has the disadvantage that for the variation of the magnifications, two components, for example the lens assembly and flat reflecting mirrors, must be moved, individually or in association with each other, precisely in positions corresponding to the magnifications, this purpose, expensive and complicated moving means of high precision is inevitably required.
Furthermore, in the conventional copying apparatuses, the parts are arranged so that the position for illuminating the original and position for starting scanning of the original are set to be constant irrespective of copying magnifications so that the copying is initiated from a leading edge of the original during copying at the magnification of 1 i.e. in equal size copying. In the copying apparatuses as described above, if the scanning exposure is effected during reduced size copying in a similar manner as in the equal size copying, the leading edge of the original is not copied in the reduced size copying, since the scanning speed during the reduced size copying is faster than that during the equal size copying. For overcoming the inconvenience as described above, a delay mechanism is generally provided for delaying the timing for scanning the original during the reduced size copying.
However, providing the delay mechanism as described above not only complicates the construction of the copying apparatus to that extent, but increases the cost thereof, since change-over means is required for the delay function.
Meanwhile, in the known slit exposure type copying apparatuses, for making the variation and distribution of the exposure amount due to alteration of the copying magnifications uniform so as to solve the problem related to uneven density of copied images, a slit forming member or slit plate forming one side of a slit is arranged to be moved into or away from an image forming light path in association with the variation of copying magnifications to alter the configuration of the slit, but such an arrangement also requires a special mechanism for the purpose, making the construction of the copying apparatus more complicated, while a particular structure is necessary for correctly setting the slit configuration, thus resulting in further complication and high cost of the copying apparatus on the whole.